bool?

public class GuestResponse
    {
        [Required(ErrorMessage = "Please enter your name")]
        public string Name { get; set; }
        [Required(ErrorMessage = "Please enter your email address")]
        [RegularExpression(".+\@.+\..+", ErrorMessage = "Please enter a valid email address")]
        public string Email { get; set; }
        [Required(ErrorMessage = "Please enter your phone number")]
        public string Phone { get; set; }
        [Required(ErrorMessage = "Please specify whether you'll attend")]
        public bool? WillAttend { get; set; }
    }

Tip As noted earlier, I used a nullable bool for the WillAttend property. I did this so that I could apply the
Required validation attribute. If I had used a regular bool, the value I received through model binding could be only true
or false, and I wouldn’t be able to tell if the user had selected a value. A nullable bool has three possible values: true,
false, and null. The null value will be used if the user hasn’t selected a value, and this causes the Required attribute
to report a validation error. This is a nice example of how the MVC Framework elegantly blends C# features with HTML and
HTTP.

5.1下测试，如果不定义为 bool? ，如果未选择，绑定后值为false，但验证也未通过，实际接收的值为“”值。